Certain astronomical events happen just once in a blue moon. We had a couple of those events earlier this year — an annular solar eclipse in May, and a transit of the planet Venus across the face of the Sun in early June. And tomorrow we’ve got another. Appropriately enough, it’s a Blue Moon.

There are all kinds of definitions for Blue Moon. It can literally mean that the Moon looks blue, which can happen when certain types of smoke particles enter the upper atmosphere. It can also refer to the fourth full Moon in a three-month period, or to the 13th full Moon in a calendar year. All of these occurrences are rare, hence the phrase “once in a blue moon,” meaning something that happens infrequently.

But over the last quarter-century or so, it’s taken on an entirely new meaning — the second full Moon in a calendar month. The Moon was full on the night of August 1st as measured here in the United States, and it’s full again tomorrow, at 8:58 a.m. Central Daylight Time — hence, a Blue Moon.

Even though the second-full-Moon-in-a-month definition isn’t old, it certainly maintains the spirit of the older definitions. Full Moons are separated by about 29-and-a-half days, so it’s difficult to squeeze two of them into a single calendar month. On average, that happens just once every 27 months. By just about anybody’s reckoning, that’s rare — something that happens once in a blue moon.

Script by Damond Benningfield, Copyright 2012

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